1. Field of the Invention
This invention relates to a method of producing inhomogeneous index free gradient index optical materials, that is, gradient index optical materials that are free of index irregularities (inhomogeneities) and have a uniform refractive index at all points on a surface thereof. The invention involves the production of infrared transmitting material the index of refraction of which changes as a function of thickness of the material.
2. Description of the Prior Art
The production in the prior art of gradient index profile) by means of the chemical vapor deposition (CVD) reaction or process has been hampered by the presence of index irregularities (inhomogeneities), or nodules, in the deposited materials. The cause of these anomalies is not well understood. It is believed, however, that these anomalies are due to gas phase nucleation. In gas phase nucleation the reacting gases react above the substrate, forming molecules of the alloy being deposited, which molecules agglomerate and eventually fall onto the substrate. Gas phase nucleation thus results in the formation of inhomogeneities, or nodules, on the surface of the substrate.
Zinc sulfide and zinc selenide are produced in a CVD reaction by reacting vaporized zinc with hydrogen sulfide and hydrogen selenide, respectively, with argon, for example, being used as a carrier gas. These materials that are so produced exhibit different indices of refraction over the useful transmission range (0.5-12 .mu.m) thereof. Furthermore, the alloy Zn S.sub.x Se.sub.(1-x) exists for 0&lt;x&lt;1, that is, for values of x that lie between zero and one. This alloy has an index of refraction that is directly related to the composition of the alloy. Therefore, it is possible by proportioning the relative magnitudes of the flows of hydrogen sulfide and hydrogen selenide to produce a material with an arbitrary gradient index profile.
The gradient index ZnS-ZnSe materials produced by the CVD process in the prior art exhibit areas of index irregularities, as illustrated in FIG. 1 of the drawings. These index irregularities produce wavefront distortions in an optical system. The amount of the distortion is related to the magnitude of the index irregularity. Therefore, in the production of inhomogeneous index free gradient optical materials, it is essential to reduce or eliminate this abnormality.